The present invention relates to a connecting structure to connect a conductive connecting tab to a battery case.
The invention relates to formation of conductive connection with high reliability, which shows high connecting strength when a conductive connecting tab is connected to the battery case. The invention relates to a conductive connecting structure, which reduces consumption of welding electrodes when the connection is formed and which contributes to stable production of a large quantity of batteries.
Various types of batteries are now used as power sources for small-size electronic devices. A nonaqueous electrolyte battery such as lithium ion secondary battery, i.e. a small-size large-capacity sealed battery, is used as the power source for portable telephone, note-book size personal computer, camcorder, etc. The nonaqueous electrolyte batteries are generally designed in cylindrical shape or rectangular shape.
In the lithium ion battery used as the power source of small-size electronic device, active material is coated on each of a positive electrode current collector and a negative electrode current collector. Then, these are wound up together with a separator between them, and this is placed in a battery case and is sealed.
In particular, a device using battery is generally provided with a battery receptacle or a battery chamber designed in form of rectangular parallelepiped. When a battery of cylindrical shape is placed in such battery receptacle, there has been such a problem that ineffective or useless space is increased. Further, the cylindrical shape is limited because of the thickness of the battery receptacle. Thus, in s small-size or thin-type device, a thin rectangular battery of prismatic shape is now used.
In a rectangular type nonaqueous electrolyte secondary battery, a battery case made of a material such as stainless steel or nickel-plated soft steel is widely used.
When a metal case made of stainless steel, soft steel, etc. is used as the battery case, a nickel-made negative electrode conductive tab mounted on a battery element is connected to inner wall surface of the battery case. The conductive tab made of nickel used as the negative electrode conductive tab shows generally good connecting performance with soft steel in resistance welding, while there have been strong demands on the improvement of the connecting performance.
When the battery is inadvertently dropped down, connecting points may be ruptured. For instance, strong force is often applied on the connecting portion between the negative electrode conductive tab and the inner wall surface of the battery case due to inertial force from the battery element. In this respect, it has been generally practiced to connect the conductive connecting tab at a plurality of points to reduce the conductive connecting resistance and to increase the strength of the connected portion.
FIG. 3 is a flow chart to explain a process to connect a conductive connecting tab to the inner wall surface of the battery case made of nickel-plated soft steel as used in the past. In this case, welding is performed at two points.
After the battery case is placed at a predetermined point on a welding apparatus, a welding unit is moved down in Step 11. Next, in Step 12, a fixing side electrode is advanced and is moved toward outer wall surface of the battery case. In Step 13, the pressing side electrode is advanced and is moved toward the surface of the conductive connecting tab. In Step 14, a point to be welded is sandwiched from both sides and welding current is supplied, and welding is performed on this first welding point.
When the welding at the first point has been performed satisfactorily, the welding unit is moved to a second welding point. In Step 16, the fixing side electrode is advanced and is moved to outer wall surface of the battery case. Next, in Step 17, the pressing side electrode is advanced and moved toward the surface of the connecting tab. In Step 18, the point to be welded is sandwiched from both sides, and welding current is supplied, and welding is performed on the second welding point.
Next, in Step 19, the pressing side and the fixing side electrodes are moved backward. In Step 20, the welding unit is moved up. The welding operation is completed, and the battery case is taken out.
In Step 14, when welding at the first welding point has been incomplete or poorly performed or the welding at the second welding point was not satisfactory, the pressing side and the fixing side electrodes are moved backward in Step 19. The welding unit is moved up, and the poorly welded product is then taken out.
As described above, in the resistance welding as practiced in the past, electric current is concentrated to the welding point by using a welding electrode having small contact area with the welding point. An electrode of about 2 mm in diameter has been used as the fixing side electrode, and an electrode of about 1×1 mm (length×width) has been used as the pressing side electrode. When the electrode with small contact area is used, electrode tip is very likely to be damaged due to heating or pressing force during the welding operation. When welding is repeatedly performed, the condition for the welding would become poorer compared with the condition in early stage of welding, and the welding electrode must be replaced as early as possible.
To solve the above problems, it is an object of the present invention to provide a battery, by which it is possible to connect the conductive connecting tab mounted on the battery element to inner wall surface of the battery case or to the battery housing at a plurality of welding points and to provide high reliability in conductive connection.